1. Field of the Invention
The present invention relates generally to communications networks, and more specifically to a telephone and facsimile communication network using the frame relay format.
2. Description of the Related At
Japanese laid-Open Patent Specification Hei-6-105115 discloses a communications network in which signals from subscriber telephones are assembled at the transmit end of a communication link into 53-byte ATM cells for transmission and disassembled at the receive end into the original format. On the other hand, signals from fax terminals are not assembled into the ATM cell format. Instead, they are segmented into blocks of predetermined length and time-division multiplexed with the ATM cells of telephone signals and transmitted. At the receive end, the transmitted signals are demultiplexed from each other to recover the original telephone signals and fax data. While this prior art can avoid the use of overheads for facsimile transmission which would otherwise be wasted by ATM cell headers, the bandwidth allocated to the facsimile communication is fixed and cannot be altered to meet required transmission speed if it is higher than the speed of transmission of ATM cells. Under such circumstances, a fallback procedure is initiated to perform fax communication at a reduced speed.
Therefore, a need exists to support the requested fax transmission speed if it is higher than the speed of transmission of telephone-related sound signals.
It is therefore an object of the present invention to provide a communication method and network where the bandwidth of a facsimile communication is determined by the remaining resource availability of a communication link after a portion of the link resource is allocated for voice-band transmission.
According to a first aspect of the present invention, there is provided a method of communication between source and destination frame relay units interconnected by a communication link. According to the method, the source frame relay unit determines, in response to a signaling message from a source terminal, whether a communication link between the terminals has a bandwidth sufficient to transmit a sound signal at a predetermined speed in a frame-relay format, and forwards the signaling message to the destination frame relay unit to establish a connection between the source terminal and a destination terminal if the link has a bandwidth sufficient to transmit the sound signal, so that a preamble tone signal is sent from the destination terminal to the source terminal. The source frame relay unit returns a busy tone signal to the source terminal if the link has a bandwidth insufficient to transmit the sound signal. Responsive to the preamble tone signal, the destination frame relay unit determines whether the link has a remaining bandwidth sufficient to support the difference between the predetermined speed of the sound signal and a speed requested by the preamble tone signal in order to transmit facsimile signals in the frame-relay format. If the link is capable of supporting the difference speed, the source and destination terminals are allowed to exchange facsimile signals at the requested speed. Otherwise, the source and destination terminals are controlled to exchange facsimile signals at a speed lower than the requested speed.
According to a second aspect, the present invention provides a communication network comprising a first frame assembler/disassembler for providing frame assembly and disassembly according to frame-relay format, a first voice-band compander connected between a first telephone switching system and the first frame assembler/disassembler, a first fax transceiver connected between the first telephone switching system and the first frame assembler/disassembler, and a first controller. The first controller is responsive to a signaling message from the first telephone switching system for determining whether a communication link has a bandwidth sufficient to transmit a compressed signal from the first voice-band compander, forwarding the signaling message to the communication link if the link has the sufficient bandwidth and returning a busy tone signal to the first telephone switching system if the link has not sufficient bandwidth. The network further includes a second frame assembler/disassembler connected to the first frame assembler/disassembler via the communication link for providing frame assembly and disassembly according to the frame-relay format, a second voice-band compander connected between a second telephone switching system and the second frame assembler/disassembler, a second fax transceiver connected between the second telephone switching system and the second frame assembler/disassembler, and a second controller. The second controller is responsive to the signaling message from the communication link to establish a connection in the second telephone switching system so that an audio signal can be sent between the first and second voice-band companders and responsive to a preamble tone signal from the second telephone switching system for determining whether the link has a remaining bandwidth which can support the difference between the transmission speed of the compressed signal and a speed requested by the preamble tone signal. The second controller allows the first and second fax transceivers to exchange facsimile signals at the requested speed if the link can support the difference speed and causes them to exchange facsimile signals at a speed lower than the requested speed if the link cannot support the difference speed.